The present invention relates to a method for solidifying radioactive wastes, and more particularly to a method for improving the long-term storage characteristics of solidified radioactive wastes comprising compact blocks which are to be disposed in transporting or final storage containers. The compact blocks comprise prefabricated ceramic tablets containing radioactive substances and an inactive matrix which continuously surrounds these tablets and is solid in its final state.
Radioactive wastes must be conditioned for permanent storage, i.e. they must be converted with the aid of matrix materials into solidified products. Such solidified products must have a high resistance to leaching of the radioactive substances by aqueous solutions.
For waste concentrates containing medium and highly radioactive wastes and/or actinides, or for fine-grained solid wastes which are present as suspensions in water or acids or for muds, ceramic matrix materials have been used, among others, to form the solidified products. The radioactive wastes are mixed with these matrix materials, are shaped and sintered to form mechanically stable bodies. For reasons of workability of such ceramic materials, the tablet shape has been selected for the ceramic solidification products. In principle, radioactive wastes that have been conditioned in this manner can be stored in suitable containers in a permanent storage facility.
There do exist, however, some considerable drawbacks with the tablet shaped solidification products. Thus, if the transporting or final storage container is damaged, the tablets may be scattered about. The danger of contamination is then augmented considerably.
Moreover, the bulk of such tablets constitutes a very large surface area. In the case of the entry of liquids, for example, water or aqueous salt solutions, the leachings of radioactive substances per unit time is relatively high.
Further, heat dissipation from the bulk tablet fill is limited.
These drawbacks can be overcome if bulk fills of ceramic tablets, whose individual volume is in the milliliter range, are solidified with the aid of a filler or binder into compact and mechanically stable blocks. The volume of these blocks is the liter range. Such fillers or binders will hereinafter be called the continuous matrix.
DE-PS No. 2,726,087 and corresponding U.S. Pat. No. 4,297,304 disclose a method for solidifying such radioactive wastes. In particular, these documents disclose a method for solidifying high and medium radioactivity and/or actinide containing aqueous waste concentrates or fine-grained solid wastes suspended in water for final noncontaminating storage in which the waste concentrates or the suspensions are subjected together with absorbing and/or hydraulically binding inorganic material, to a ceramic firing process so as to produce a solid sintered body.
The method comprises a plurality of steps, including (a) treating the waste concentrates or suspensions by evaporation, to form an evaporate having a water content in the range between 40 and 80 percent by weight and a solid content whose metal ion and/or metal oxide concentration lies between 10 and 30 percent by weight of the evaporate being formed, and adjusting the pH of the evaporate to between 5 and 10; (b) kneading the evaporate obtained from step (a) with a clay-like substance containing a small quantity of cement, or with such a clay-like substance or mixture of a clay-like substance with a small quantity of cement containing an additive for suppressing the volatility of alkali metals or alkaline earth metals which may be present in the evaporate and/or an additive for suppressing the volatility of any decomposable anions which may be present in the evaporate selected from sulfate, phosphate, molybdate and uranate ions, at a weight ratio range of evaporate to clay-like substance of 1:1 to 2:1, the clay-like substance being at least one substance selected from pottery clays, stoneware clays, porcelain clay mixtures and kaolin; (c) producing molded bodies from the kneaded mass obtained in step (b); (d) heat treating the molded bodies, including drying at temperatures between room temperature and 150.degree. C., calcining at temperatures up to 800.degree. C., and subsequently firing at temperatures between 800.degree. and 1400.degree. C. to form practically undissolved mineral phases; and (e) enclosing the molded bodies containing the fired mineral phases on all sides in a dense, continuous ceramic or metallic matrix.
The molded bodies of step (d) can be comminuted to a grain size range of about 1 to 10 mm, and thus be in the form of small particles or chips before being enclosed in the matrix of step (e).
The continuous matrix can be a fired ceramic produced from at least one clay substance and at least one cement. It has been found, however, that if a continuous ceramic matrix is used produced from at least one clay-like substance, e.g. from the group including pottery clays, porcelain clay mixtures or kaolin, and cement, and particularly if this mass has been processed into a fired ceramic, the solidified product does not have the desired properties. No clay-like material, with or without the addition of cement, has been found thus far for use as a continuous matrix which, in its sintered state, has a coefficient of thermal expansion which is at least very similar to that of the ceramic tablets and which shrinks uniformly and tightly onto the ceramic tablets during firing so that in the past solidified blocks were obtained which were penetrated by extensive cracks. The cracks permitted the access of liquids into the interior. Moreover, the mechanical stability of such blocks was limited.
These drawbacks could also not always be overcome by the use of a hot pressing technique. In contrast to mixtures of particulate bodies which can be optimally compressed and sintered, there are limited possibilities for compressing mixtures of sinterable, clay-like or ceramic powders. The compression limit is reached when the ceramic tablets contact one another and support themselves on one another. Beginning with this state, the pressure no longer acts on the ceramic powder disposed in the interstices. Sintering then takes place practically without pressure, i.e. compression occurs only by the shrinkage caused by the sintering process. Thus, the same or similar results can be expected as in the above-mentioned process of DE-PS No. 2726087 and U.S. Pat. No. 4,297,304 which is a pressure-free sintering process.
If it is attempted to effect compression beyond the stated limits, this unavoidably results in destruction of the ceramic tablets. At the customary sintering temperatures, the ceramic matrix material does not flow so plastically that it is able to cover the resulting fragments on all sides, and accordingly the pressure surfaces remain practically open. One advantage of embedding the ceramic tablets in a matrix, namely the reduction of the surface area accessible to leaching when the transporting or permanent storage container is damaged, is thus eliminated.
A more extensive compression than described above without the danger of destruction of the ceramic tablets can be realized if it is assured, by a high mixing ratio of ceramic powder to ceramic tablet, that in the compressed state there will always be matrix material between the ceramic tablets. Independently of whether this state can be realized with sufficient reliability under conditions applicable to working with highly radioactive substances, there exists the drawback that the volume of the container holding the block with the solidified tablets cannot be utilized optimally with respect to the tablets, since the matrix material enforces a certain "spacing" of the tablets. This drawback is connected with the fact that unavoidably expensive permanent storage volume must be occupied with inactive substances.